In a conventional printing press, for instance an offset press composed of several printing units, each unit for printing in one colour, there is a cylindrical roller which is partially immersed in each fountain pan of each printing unit, and which is rotatable to pick up liquid from the pan and distribute a uniform film of liquid to a mating roller. A liquid circulation and make-up system must be provided. Constantly circulating the liquid in the fountain pan is desirable to maintain a minimum liquid temperature gradient across each pan, to prevent growth of algae and to flush foreign particles to be filtered off the liquid. Each fountain pan is provided with a stand-pipe and the liquid pumped into the pan rises to the height of the stand-pipe and drains through the stand-pipe to a return conduit. It is very important to prevent introduction of air into the liquid at any stage of the liquid circulation system. Air causes foaming, and this can produce unpriming of the circulating pump and also the presence of foam in the fountain pans has a noticeably adverse effect on the quality of printing.
U.S. Pat. No. 4,300,450 dated Nov. 17, 1981--to Gasparini and entitled: PRINTING PRESS LIQUID CIRCULATING SYSTEM INCLUDING AN ANTI-FOAMING DEVICE describes a printing press liquid circulation system of the type in which a venturi is used to circulate the liquid through a filter immersed in the liquid of a reservoir. A pump also immersed in said liquid is used both to circulate liquid through the venturi and into the fountain pan. Due to this arrangement, the pump must be of a much larger size than it would be necessary if it was used only to supply the liquid to the fountain pan of the press. Also, the liquid reservoir, together with the venturi, must be maintained at a certain relative level with respect to the fountain pan, in order for the venturi to operate properly. This system is therefore not adapted to be used in conjunction with several fountain pans disposed at different levels.